The present invention relates generally to improved tone scale mapping in electronic cameras and, more particularly, the invention relates to an improved method and apparatus for increasing tone differentiation in a digitally recorded image without increasing a number of available discrete intervals.
The human visual system perceives high spatial frequencies of an image as sharpness and low spatial frequencies as tone scale. That is, sharpness of an image is generally a function of how well the edges of an object in an image are perceived whereas the tone scale is a function of how the image's color and its gradual variation over the extent of the image are perceived. Since the human visual system perceives sharpened images much more favorably than unsharpened images, most electronic imaging systems which process recorded images attempt to provide sharpened images for human viewing.
In electronic imaging cameras, charge coupled devices (hereinafter "CCD"'s) capture the image and transform the image into an analog signal. The analog signal is then transformed according to a gamma correction curve. The transformed image is passed through an analog-to-digital converter ("A/D") creating a digital signal representative of the image.
Historically, video camera recordings were displayed on cathode ray tubes ("CRT"'s), which have a characteristic gamma curve. Another gamma correction curve was employed in the video camera to create a system tone scale in the recording which is pleasing to the human eye as shown on the CRT. Conventional photography has also made use of gamma correction curves in the form of density-exposure curves, commonly known as D-log H curves in the art, by adjusting chemical balances in film and processing. This is analogous to the video use of gamma curves in that the film is now being used to alter the tone scale perception to create tone scales that are more pleasing to the human eye in a hard copy of the image.
These two examples of cameras are utilizing gamma curves to correct for an output device to make the image as displayed on the output device more pleasing to the eye. In an electronic still image camera the output device can vary from a film recorder wherein conventional photographic film is utilized, a laser printer where a color image is rendered upon paper, or the image may simply be transmitted to a computer where it is displayed upon a CRT, inter alia. A gamma curve for all of these scenarios must then be designed into the camera which compromises between these output devices.
An additional problem in electronic still image cameras is that the image is ultimately digitized through the A/D. The problem here arises in that tone scale changes may be lost if discrete intervals of the digitization are too large. Large steps in digitization create contouring seen in an image as uneven steps between tone which are noticeable to the human eye. In other words, where tones are changing rapidly, the divisions between the digitized words may not be close enough together to represent the large changes in tone scale smoothly without contours. Therefore, the eye is able to see the changes reducing how pleasing the image looks to the eye. This is called contouring. The size of the discrete interval can be decreased by increasing the number of bits of the digitization. As the number of bits increases though, so does the cost of the circuitry to perform the digitization.
Therefore, it is an object of the invention to present a gamma curve that takes into consideration various output devices.
It is another object of the invention to reduce the cost of manufacturing still image cameras by minimizing the number of bits in the A/D.
It is still another object of the invention to reduce contouring in an electronic still image thus avoiding visible steps between quantized levels of the image. These and other objects of the invention will be obvious and will appear hereinafter.